Interesting question
Solar power is everywhere, is free and is best for earth's health too, so why no solar cells in cellphones.
The issue could also be found when we say why can't we have solar cars or solar panels installed at every roof.
Solar panels have their own disadvantages
1.Cost : solar panels are made from silicon and germanium anon with other expensive metals. Some are rarely available.
This is why solar panels cost too much.
2. Space : solar thermal absorbing needs a large space to make sensible amount of energy. A cellphone is rather preferred compact.
3. How long in a day do you sit in sun?
4. A wire attached to your device vs its out in sun, you decide.

With time inventions like fast charging or wireless charging would enhance enough to not make us sit like a dog on a leash for hours.

Solar cells aren't efficient enough to put on a cell phone and use. To equal the same output as say the battery you have in a Q10, you would need a solar cell about 24 square inches, which would not exactly fit in your pocket now would it, and since it would have to be facing the sun all the time to get maximum benefits. Let's not forget about the back up powersource you would need because the sun isn't out 24/7/365 shining on that solar cell you want for your phone which would negate that thing about being so echo friendly.

There will probably be some small startup that will produce a phone with the back of the phone as a solar cell. Flip the phone to trickle charge. Solar cells will have to be thinner and more efficient before this happens otherwise the phone will be thicker and take hours longer than a plugin or wireless charging solution.

Phones run from batteries, because they need a lot of power in a short period - more than you can comfortably get from most solar panels the size of a phone. Think of the battery as a lay station that holds the power for when it needs it.

A big part of this is the radio, GPS, cpu, display, etc. To circumvent this you could use a less draining device like a Lightphone quite well with a solar panel

That said - you could use a solar charger though to charge the battery right now if you want, the units are available. Powering it off the grid is just a user's choice at that point.